Full movie description "Hinterland Episode #1.3":

A body is found in the depth of a flooded quarry near the remote village of Penwyllt. The investigation tries to discover who he was and if his death was an accident.

Reviews of the Hinterland Episode #1.3

We are given some hints about Mathias's past. He is devastated when he
looks at the pictures of a couple little girls. Obviously, when things
involve children he lets up from his cynicism. Hopefully, we will know
at some point. The murder involves a young man, seemingly liked by
everyone, who has drowned, but not where he was found. His body was
moved. We are in constant contact with a recluse who is a prime
suspect. A tense marriage where a lonely little boy is involved. Then
there are others who are scratching a living from the land. The men are
big and bearded and protective of one another. Once again, despite his
incredible police work, Mathias is routinely confronted by that
supervisor. He never seems to do anything but grunt. Is there some
history?